1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a magnetic layer having a thickness of 1.0 .mu.m or less. In particular, the present invention relates to a magnetic recording medium which has excellent electromagnetic conversion characteristics and durability, and is suitably used in an apparatus which records and reproduces digitized signals by a helical scan system.
2. Description of the Related Art
A magnetic recording medium is usually produced by coating a magnetic paint comprising magnetic powder, a binder component, an organic solvent and other necessary components on a non-magnetic support, such as a polyester film, and drying it. To satisfy the requirement for high density recording, the thickness of the magnetic layer is gradually decreased.
Recently, with the progress of signal processing and digital circuit technologies, the signals are digitized and recorded by a video recorder. Presently, since a thickness of the magnetic layer is still thick, unsaturated digital recording is employed. But, for recording and reproducing the digital signals, saturated digital recording which can be overwritten as in the case of a magnetic disc apparatus for a computer is preferred. To effect the saturated digital recording the thickness of the magnetic layer is preferably made thin.
However, when the magnetic layer is made thin to increase the recording density and improve the overwriting characteristics, surface conditions of the non-magnetic support tend to influence the properties of the magnetic layer, so that the electromagnetic conversion characteristics deteriorate, for example, the reproducing output is lowered as the thickness of the magnetic layer is decreased.
To prevent influence on the surface conditions of the non-magnetic support and to improve surface smoothness of the magnetic layer, an intermediate layer comprising a non-magnetic powder is provided between the non-magnetic support and the magnetic layer. However, while the conventional intermediate layer can prevent the influence on the surface conditions of the non-magnetic support, pin holes and/or coating streaks are formed and the durability is not improved sufficiently.
From the rheological view point of the intermediate layer when a magnetic paint is coated, it in order to increase the durability a improvement of the composition of the intermediate layer and/or coating method have been attempted. For example, it has been proposed to provide a magnetic layer having a thickness of less than 1.0 .mu.m while the intermediate layer is still wet, whereby the electromagnetic conversion characteristics and also the durability are improved as seen in U.S. Pat. No. 5,258,223.
However, while the formation of pin holes and coating streaks on the magnetic layer surface was prevented, the surface smoothness of the magnetic layer was not improved satisfactorily, and the electromagnetic conversion characteristics may be worse than those of the magnetic recording medium in which the magnetic layer was directly formed on the non-magnetic support, in the absence of the intermediate layer.